1. Field of the Invention
This invention relates to High Frequency Microwave Oscillators, and in particular, to a MIC (Microwave Integrated Circuit) Dielectric Resonator Stabilized Oscillator (DRSO) which is electronically tuned by an inductively coupled Varactor Diode via Microstrip, resident on a plane other than the substrate layer supporting the oscillator microstrip circuit.
2. Description of the Related Art
Microwave oscillators form an important part of all microwave systems such as those used in RADAR, communication links, navigation and electronic warfare. With rapid advancement in technology there has been increasing need for better performance oscillators. The current emphasis is on low noise, small size, low cost, high efficiency, high temperature stability and reliability. Conventional low-frequency resonance circuits consisting of the usual coil and capacitor are not practical at microwave frequencies, because the required values of capacitance and inductance are extremely small. For this and other reasons, it is necessary to select a different form of resonant circuit in the microwave frequency range.
Early in the history of microwave application, it was discovered that in order for a component to resonate at higher frequencies, a hollow closed metal box of nearly any size or shape could be made to resonate in the same manner as a conventional LC circuit at lower frequencies. One advantage in using microwave frequencies is that the higher the frequency, the smaller the component dimensions.
Dielectric resonators provide temperature stable, high Q resonant elements suitable for microwave integrated circuit (MIC) applications. A microwave oscillator using a barium tetratitanate ceramic material (Ba.sub.2 Ti.sub.4 O.sub.9) resonator which is integrated into the feedback circuit of a transistor has important advantages. Dielectric resonators provide a significantly simpler oscillator than any alternative microwave generator giving comparable stability at a much lower noise and size. The dielectric resonator typically has a dielectric constant of between 38-40.
In addition to dielectric resonators, microwave stabilized oscillators use planar circuits to designate conductor networks. These planar circuits are deposited over one or both faces of an insulating plate, such as the dielectric substrate. The dielectric resonator and the planar circuits are both key components for the production of microwave integrated circuits (MIC).
One example of a prior art construction of an electronically tuned dielectric resonator stabilized oscillator (DRSO) is shown in FIG. 1. The entire oscillator system 10 is enclosed within a metal conductive material container which is not shown. Along the bottom surface of the container an alumina (Al.sub.2 O.sub.3) substrate 12 is provided, which isolates the dielectric resonator 16 from direct contact with the metal box that contains the oscillator system 10. Mounted above the alumina substrate 12 is a fused silica (Si O.sub.2) spacer 14, upon which is mounted the dielectric resonator 16. Typically, the dielectric resonator 16 is made from barium tetratitanate (Ba.sub.2 Ti.sub.4 O.sub.9).
Etched along the upper surface of the alumina substrate 12 and immediately below the fused silica spacer 14 are microstrips 18 and 24. Microstrip 18 is associated with and coupled to the gate of a common source FET transistor 20. The output signal is derived at the drain of the FET transistor 22. The microstrip 24 is coupled to a varactor diode 26 and is independently powered by the electronic tuning voltage source 28. A rather broadband noise signal of high frequency is provided to the microstrip 18 and is selectively tuned by the dielectric resonator 16 through magnetic field coupling. Frequencies of such microstrip and dielectric resonator circuits may center at a selected resonant frequency in a range anywhere from 1 to 35 gigahertz (GHz). The geometric configuration of dielectric resonator 16, and its associated dielectric constant, provide selection for a resonant frequency for this oscillator system. The magnetic field of microstrip 24 of the varactor diode 26 circuit also may be coupled to the magnetic field of the dielectric resonator 16 in order to electronically tune the signal selected by the dielectric resonator 16 for transmission along the microstrip line 18 to the output FET transistor 20. In this prior art configuration, it will be noted that both the microstrip line 24 and 18 are etched on the same alumina substrate 12.
Another example of a prior art tunable microwave oscillator is U.S. Pat. No. 4,724,403, issued to A. Takayama, assigned to Alps Electric Co. of Japan. In this patent, there is disclosed a microwave oscillator 10 having a metal cover 5. The resonant frequency housing 7 encloses a dielectric resonator 4, while an adjustable screw 6 is used to mechanically tune the oscillator. Other examples of such mechanically tuned dielectric resonator microwave oscillators include U.S. Pat. No. 4,728,913 to Y. Ishikawa (assigned to Murata Mfg. Co. of Japan) and U.S. Pat. No. 4,521,746 issued to E. J. Hwan, et. al. (assigned to Harris Corp. of Melbourne, Fla.). In both these patents, elaborate mechanical tuning is used to increase the range of tuning performance of a dielectric resonator that is housed within a conductive enclosure. Mechanical tuners, while provided a wide range of tuning do not necessarily provide the fine tuning that is necessary for certain microwave applications, such as the use of a microwave oscillator as a local oscillator in a microwave radar receiver system. Fine tuning requires electronic tuning.
Problems have arisen with the prior art construction shown in FIG. 1. By etching both the microstrips lines 18 and 24 on the same alumina substrate, the varactor diode 26 and its associated microstrip 24 have the effect of increased loading of D.R. quality factor thus increasing the close-in noise of the oscillator system.
In the prior art, attempts to improve on the design shown in FIG. 1 have been made in U.S. Pat. No. 4,683,447, to Ashok K. Talwar, et. al., on a patent issued July 28, 1987. In this design, disclosed in the '447 patent, the varactor diode 22 (FIG. 3 of the '447 patent), is electrically connected in a loop 24 with an RF bypass capacitor and is imbedded within a dielectric resonator (36 and 38) which is made from a first and second dielectric substrate and face each other along the plane of the loop 24. As in Prior Art FIG. 1, this patent teaches the placement of the electrically conductive loop 24 along the upper surface 54 of the substrate 56, in the same plane as the microstrip transmission line 10 associated with the oscillator system and its output transistor 8.
The article, NOVEL TECHNIQUES FOR ELECTRONIC TUNING FOR DIELECTRIC RESONATORS, by A. N. Faar, et al.; Proc. 13th European Microwave Conference, (Germany) pp. 791-796, (1983), presents an alternative design for varactor tuning of a dielectric resonator as shown in FIG. 7 on page 796. Here, the authors employ a method of voltage control varactor tuning, where the varactor circuit is located above the resonator in order to control the resonator field. The authors show a circular microstrip associated with the varactor diode tuning circuit mounted on top of a quartz spacer, which resides above the dielectric resonator. The dielectric resonator itself is mounted directly on the same substrate as an oscillator microstrip line. This article claims a demonstrated tuning range of 0.75%.
This publication also teaches that an alternative acceptable configuration would be to position the varactor circuit and associated bias line adjacent to the microstrip feed line for the dielectric resonator oscillator on a common substrate. By recommending such a design, the authors overlook the problems inherent when both the oscillator microstrip and tuning microstrip lines are coupled on the same substrate to the dielectric resonator. Furthermore, by having the dielectric resonator reside directly on the alumina substrate, a considerable loss of resonant Q occurs.
The current family of oscillator (feedback) topology, as shown in the Talwar patent and Prior Art FIG. 1, illustrates the inherent difficulty due to physical circuit layout to implement the varactor diode tuning circuitry on the same MIC substrate.
One of the major concerns with the operation of any dielectric resonator stabilized oscillator system (DRSO) is the characteristic FM noise close to the resonant frequency (less than 100 (khz)). In order to optimize FM noise characteristics the dielectric resonator is placed in a location away from the microstrip coupling lines 18 and 24 and the substrate 12 ground plate. In FIG. 1 with the varactor coupling circuit 14 etched on the same substrate as oscillator coupling circuit 18, the adjustment of oscillator resonator loaded Q and electronic tuning sensitivity are mutually affected. As the spacer thickness is increased to improve the resonator loaded Q, the electronic tuning sensitivity is decreased, requiring a new substrate printed circuit geometry to gain back the lost sensitivity. This may take many iterations before the desired results are achieved.
Also, the prior art configuration requires flexibility in the orientation in the varactor coupling circuit with respect to the main oscillator printed circuitry. This is the case since the varactor coupling circuit and the resonator oscillator coupling line lie in the same plane. Such constraints limit size reduction capability and package of versatility. Thus, in the prior art FIG. 1, consideration of the coupling effects of both the microstrip line 18 associated with the main oscillator circuit and the microstrip line 24 associated with the varactor diode 26 must both be taken into account. This continual adjustment for purposes of oscillator system optiminization makes the manufacturing process of the desired MIC oscillator hybrid circuit costly and difficult.